This invention relates to the fabrication of optical waveguides and, more particularly, to the fabrication of stress induced optical waveguides in bulk crystalline materials.
A variety of optical waveguide structures have been proposed and manufactured in semiconductor materials. These include guiding structures made from thin films and etched ribs, as well as guiding regions made from materials doped with metallic atoms. A more recent development has been the discovery that semiconductor doping can be combined with a material's elasto-optical properties to produce waveguide structures.
It is known that light can be confined along an axis normal to adjacent epilayer surfaces in some semiconductor materials. This is typically accomplished by epitaxial growth of n.sup.- type material having a given free carrier plasma density (on the order of 10.sup.18 excess electrons/cm.sup.3) on n.sup.+ type material having a much lower free carrier plasma density (on the order of 10.sup.15 excess electrons/cm.sup.3). A free carrier differential between adjacent epitaxial layers of semiconductor material creates a change in the refractive index seen by light traversing the interface region between the layers. Light entering the interface region within the appropriate solid angle, is reflected back into the semiconductor material having the higher index of reflection leading to the confinement of one or more modes of light in that region. Therefore, a waveguide structure can be constructed from two layers of n.sup.+ type semiconductor material positioned on either side of a layer of n.sup.- type material for the same semiconductor material.
It is also known that one of the epilayers can be replaced with a thin strip of metal bonded to the surface of the n.sup.- type epilayer. The metal strip alters the free carrier density just below the surface of the semiconductor material which also confines light as in the n.sup.+ layer arrangement. However, confinement of light laterally or parallel to the epilayer interfaces requires additional interface structures or alteration of material properties within the epilayer to vary the refractive index within the epilayer.
In order to obtain lateral confinement, L. D. Westbrock et al proposed the use of stress in the n.sup.- epilayer material to provide variations in the refractive index. See "The Strain Induced Waveguiding in GaAs Epitaxial Layers at 1.15 .mu.m" L. D. Westbrock et al, Electronics Letters, Vol. 15, No. 3, Feb. 1, 1979, pp. 99-100 and "Photoelastic Channel Optical Waveguides in Epitaxial GaAs Layers" by L. D. Westbrock et al, Electronics Letters Vol. 16, No. 5, Feb. 28, 1980, pp. 169-170. Stressing a material alters the dielectric constant of the material through the photo-elastic effect similar to the change in the dielectric constant affected by a voltage through the photo-optic effect. This is discussed in relation to semiconductor lasers in "Photoelastic Waveguides and Their Effect On StripeGeometry GaAs/ Ga.sub.1-x Al.sub.x As Lasers" by P. A. Kirkby, P. R. Selway and L. D. Westbrock, Journal of Applied Physics, Vol. 50, No. 7, July 1979, pp. 4567-4579.
In the case of stripe-geometry lasers, it was discovered that commonly used surface coatings can cause weak waveguiding regions in n.sup.- type epilayers due to induced stress. Westbrock et al employ a thin strip of silicon dioxide or a metal film deposited along a planar epilayer of n.sup.- type gallium arsenide grown on a heavily "doped" n.sup.+ gallium arsenide substrate. The vertical trapping of the light to the thin n.sup.- epilayer is achieved as described above because of the free carrier distribution. Guiding in the lateral or horizontal direction is provided by stress induced in the n.sup.- epilayer as a result of the difference in the coefficient of expansion between the film and the gallium arsenide. A thin film firmly bonded to the surface of GaAs transfers stress into the material as the two expand or contract. Westbrock et al achieve stress on the order of 1.times.10.sup.8 to 5.times.10.sup.9 dynes/cm.sup.3 with this technique.
The magnitude of the change in the index of refraction induced in the n.sup.- epilayer is proportional to the difference between the temperature of deposition of the film and the operating temperature of the waveguiding material. Relying on the disparity in material thermal expansion alone, however, does not provide large enough variations in the index of refraction in the n.sup.- epilayer to achieve waveguiding without the assistance of the n.sup.+ epilayer structure in confining light close to the surface of the n.sup.- epilayer where it can sample the index difference.
However, the n.sup. +type of epilayer structure tends to be lossy, since a heavily doped n.sup. +GaAs substrate is strongly absorptive and rapidly attenuates the confined light. Therefore, even though waveguiding is achieved, the transmission losses of the guide quickly dissipate optical energy and limit usefulness.
This technique is limited to epilayer structures in materials that lend themselves to the appropriate doping levels. Bulk crystalline materials with desirable optical properties cannot be used by this technique. Some materials, such as ADP or KNbO.sub.3, are extremely difficult to grow in epilayer structures.
It is also desirable to be able to use a broader class of materials, especially nonlinear materials, that allow increased optical intensities and unique optical processing abilities in addition to simply guiding optical radiation. There may also be manufacturing and cost restrictions of epilayer materials due to the required precision for assuring single mode operation.
Accordingly, a principal purpose of the present invention is to provide a method for producing optical waveguides and waveguide devices in bulk crystalline materials which are more efficient and less lossy than previously demonstrated stress waveguides.